Fish passes, fish locks, or fish elevators have been employed to overcome natural obstacles and man made obstacles such as dams. Fish passes, usually comprising a series of pools of water constructed in the form of steps, have limitations. They are costly to install and must be laid out on a gradual rise, each step limited to the height that the fish are able to naturally overcome. These types of by-passes also require diverting part of the waterflow thereby losing a percentage of the total possible hydro development. In cases of very rugged terrain it is, at times not practical to install these known types of systems and the river is made impassable to the fish.
Various types of fish elevators are also known. These elevators usually comprise a buoyant cage or crowder in which the fish are either raised above or lowered below the obstacle. These elevators have limitations in that they usually require a type of pump to assist in raising or lowering the buoyant case. For example, U.S. Pat. No. 3,377,805 discloses a type of fish elevator wherein an external pressure system is required to supply additionally needed water so as to raise the buoyant cage to the top of the passageway.